Molten steel is produced into steel products, such as slabs, blooms, billets or the like, through a continuous casting process. In the continuous casting process, molten steel flows from a tundish through a submerged nozzle into a mold, and is cooled through passage through the mold to produce a steel product, for example, a slab. When the molten steel passes through the submerged nozzle into the mold, argon gas is introduced to the molten steel in order to prevent the molten steel from being solidified in the submerged nozzle. When the molten steel passes through the mold, a solidified shell is formed along the surface coming in contact with the mold. If argon gas is trapped in the solidified shell, it will cause pinhole defects immediately below the surface layer of the resulting slab. The pinhole defects may evolve into line defects in the resulting hot-rolled and cold-rolled coils.
The background art of the present invention is disclosed in Korean Patent Laid-Open Publication No. 10-2005-0021961 (Mar. 7, 2005; entitled “Method for Producing Ultra-Low Carbon Steel”).